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Electrical & Wiring·Lesson 11 of 34

Branch Circuit Breakers and Fuses

How each device's circuit is protected, which breakers are legal, and how to size them.

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Branch circuits

Every output channel of the PD is a branch circuit. Rule R610 requires every circuit (except the roboRIO and radio circuits covered by R615/R617) to be protected by a breaker or fuse, and R621 requires each branch to be protected by one and only one breaker/fuse sized per Table 8-3. These auto-resetting breakers protect both the wiring and the motor controller from sustained overcurrent.

Legal breaker types (R619)

Only specific breakers may go in a PD:

  • Snap-Action VB3-A series or AT2-A (terminal style F57), 40A or lower
  • Snap-Action MX5-A or MX5-L series, 40A or lower
  • REV Robotics ATO auto-resetting breakers, 40A or lower
  • CTR Electronics ATO breakers, 40A or lower (added for 2026)
  • ATM breakers up to the value permitted for fuses per R620

These are auto-resetting thermal breakers: when they trip from overcurrent they cool down and reset themselves automatically rather than needing a manual reset.

Sizing by Table 8-3 (R621)

The maximum protection depends on what the channel feeds:

  • Motor controllers: up to 40A (one breaker each).
  • Custom circuits: up to 40A (no quantity limit).
  • Spike relay / Automation Direct 25A relay, PCM/PH with compressor, Servo Power Module/Servo Hub: up to 20A.
  • Additional non-radio VRM / non-compressor PCM/PH: up to 20A (three total).
  • roboRIO and radio: a 10A fuse/breaker on a dedicated channel (R615/R617).

A common mistake is putting a 40A breaker on a circuit wired with thin wire - the wire would melt before the breaker tripped. The breaker must match both the device and the wire gauge.

Fuses vs breakers (R620)

Low-current channels often use fuses instead of breakers. R620 limits fuses to automotive blade types: ATM fuses for the PDP and for the PDH (up to 15A, with a single 20A exception for a PCM/PH compressor), and ATC/ATO fuses (10A or lower) for the PDP 2.0. Unlike breakers, a fuse does not reset - it blows once and must be replaced, so keep spares in your pit kit.

Practical tips

  • Label each channel so you know which breaker feeds which device.
  • Carry a full set of spare breakers (40A, 30A, 20A) and 10A fuses to every competition.
  • A breaker that trips repeatedly is a symptom - investigate the mechanism or motor for a jam or stall; do not just up-size the breaker.

Sources

Key takeaways

  • Each branch circuit needs exactly one breaker/fuse sized per Table 8-3 (R610/R621).
  • Legal breakers are Snap-Action VB3/AT2/MX5, REV ATO, CTR ATO (2026), or ATM - all 40A or less (R619).
  • Match the breaker to BOTH the device and the wire gauge; a 40A breaker on thin wire is dangerous.
  • Motor controllers/custom circuits get up to 40A; relays/PCM/PH/servo up to 20A; roboRIO and radio use 10A. Fuses don't reset, so carry spares.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.Which Snap Action breaker is used in the large (40A) channels of the Power Distribution Panel?

2.The amperage printed on an FRC branch circuit breaker refers to what kind of current?

3.What is a key advantage of the auto-resetting (self-resetting) branch breakers like the REV/CTRE ATO breakers?

Answer every question to submit.